text:
Why not more Max 8 accidents?
Why did "only" two Max 8 accidents occur? Before the grounding, there must have been at least dozens but surely 100s if not 1000s of flights with this model. How come the vast majority of those flights didn't experience any problems?

https://www.tailstrike.com/291018.html
https://www.tailstrike.com/100319.html
These links provide transcripts of the CVR from both flights. It's worth a read, even if it is a morbid topic.

With a well functioning angle of attack sensor the 737 MAX was safe. Only a malfunction which let the sensor report a higher angle than real would result in the repeated attempt of the MCAS to trim the airplane nose down.
Such a malfunction is a rare event (10 per year across all types flying in the US). This should explain the many uneventful 737 flights. However, to rely on only one sensor was criminally negligent. A safe design would have redundancy built in. Ironically, the 737 MAX has such a second sensor (just opposite of the one which fed the MCAS) but the software did not crosscheck the readings.

Additional remarks
Now there have been so many comments, and they all have been caused by my rather cursory description of the problem, that I better expand this answer so people will not feel a need to add more comments.
MCAS was already introduced with the Boeing 707 and the KC-46 (tanker version of the 767), but would only add nose-down trim once if a prescribed angle of attack is exceeded.
With the more powerful LEAP engines of the 737 MAX, pitch stability at high angle of attack and high thrust would become negative, something that would make any airplane immediately impossible to certify under current regulations for civil aircraft. The more forward position of the intakes and the high suction on the intake at high angle of attack would had required a larger horizontal tail if proper design rules would had been applied.
But Boeing is run by ex-McDonnell-Douglas management where cost-cutting is more important than applying proper design rules. A larger horizontal tail would result in structural changes throughout the rear part of the airplane, require new jigs and tests and a much more complex certification. Simpler solutions did not work out, either: Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. So the instability had to be fixed in software.
The result was a system that read inputs from a single sensor, and under the right conditions (flaps up and autopilot disengaged) changed pitch nose-down by 2.5 degrees stabilizer rotation when a specified angle of attack was exceeded and repeated this mindlessly after five seconds if the angle of attack was still too high. No provisions had been made for a defective sensor. Concerns by engineers and test pilots about the hazards resulting from a sensor failure were ultimately dismissed. Eventually, Boeing officially assumed that pilots would respond to an unexpected MCAS activation as if it were a runaway stabilizer trim event, and do so within four seconds.
All 737s have two AoA sensors, but the MAX MCAS relied on only one sensor at a time, switching between the left and right one each flight. By the way, the KC-46 MCAS, although less powerful, uses the inputs of two sensors and trims the stabilizer by just 0.6°, and only once per activation.
Next, in order to limit the amount of pilot training for the new type to Level B (no simulator), the MAX should feel to the pilot like a next generation 737. This made it possible to fly the MAX with the type rating for a regular 737, a big selling argument to airlines which already operated the 737. MCAS helped to achieve this, and information about its details was not part of mandatory training (which consisted of a two hour iPad tutorial). By repeating the pitch trim change, the MCAS had become safety critical where earlier versions were not. Nevertheless, information about it was hidden and most pilots did not even know of its existence after their transition to the MAX.
Another factor was the insufficient funding and regulatory capture of the FAA in the last decades which had less personnel to check new designs, so it gladly handed much of the testing for certification to the manufacturers: Boeing Authorized Representatives are Boeing employees who are granted special permission to represent the interests of the FAA and to act on the FAA’s behalf in validating aircraft systems and designs’ compliance with FAA requirements. The core function of the certification process, to let a second pair of trained eyes look over the new stuff, was outsourced to the same people who had authorized those designs. Also, the capacity of MCAS was misrepresented to the FAA as an addition to speed trim in order to avoid more scrutiny and testing. Later investigations found a disturbing trend of FAA management treating operators favorably and overruling the work of technical experts.
As usual, several opportunities to avoid what happened were missed and several causes conspired to make MCAS the trap it became.

And if the software did cross check it, then what? You cannot use voting with just two inputs. Of course  the MCAD would then have to be shut down.

@Abdullah Absolutely correct. But this is better than to blindly trust a faulty sensor, isn't it?

Agreed. But it's not good rocket science.

@Abdullah You can always add some logic that computes an alpha estimate from speed, weight, load factor and altitude and compare this to the measurement. Then, when the readings disagree with the estimate, discard the readings. But now you are at the mercy of your pitot-static tube and the computer, and we all know those are hardly immune to erroneous results. Too much rocket science only gets you deeper into that pit of multiple failure modes which all have to be accounted for.

@PeterKämpf Strangely enough Airbus even does it to check weight after take-off (warning is given if the weight estimated from airspeed, AoA and vertical acceleration differs from what is entered in the FMS by more than 4t), but it still does not use it to detect double fault in airspeed or AoA sensors. Even after two accidents and a couple of incidents demonstrated it would actually be useful.

IMHO the criminally negligent part was not the reliance on a single sensor as much as the MCAS system repeatedly adding more nose-down trim, to a ridiculous degree, without getting any response from the AoA.   The cycle is what doomed planes.  If MCAS had shutoff and reported, "AoA is not responding to Nose-Down Trim commands.  Manual Control is required!", many lives would different.

@abelenky Yes, and this single-correction mode was what MCAS initially did. Only with the MAX was this extended. You might as well blame Boeing management because they outsourced this software to India where it was done by programmers without the proper education in flight dynamics and safety culture. And consecutively waved through by the FAA. So there should have been several steps where this nonsense could have been stopped. But Boeing management knowingly removed all the stops. I shortened the long story a bit in my answer.

@abelenky If the system had rendered control to the pilots after one correction step, the MAX would not be certifiable. Remember, this was about pitch instability at low speed and high thrust. The extension of the MCAS was on purpose to pitch the airplane down enough so that pilots with manual control would never experience this instability. Relying on a single sensor without considering that this sensor could be faulty was the mistake at the core of this.

Just curious; would the "AoA disagree" alarm have been the life saver the media made it out to be? I mean, given how the pilots didn't know about the MCAD, I can easily imagine them going "so what?".

@Abdullah: First of all, this warning did not work in 80% of the planes. For the remaining fifth it probably added another attention grabber just when pilot workload peaked. With proper training and knowledge about the details of MCAS, yes, it would had helped, but without that knowledge it would simply had added to the confusion and stress.

As I expected. And now it's mandatory. Sigh.

@Abdullah But by now it will be close to impossible to find a commercial pilot who does not know about MCAS. So it should help.

Most accidents in aviation are not the result of a single failure or mistake, but result from many contributing factors. If one of those had been eliminated, the accident would not have happened (or would have been less severe). This is often called the Swiss cheese model:

(image source: Wikipedia)
Only when all holes in the cheese line up, can the line pass through. In aviation this means, only when all contributing factor occur at the same time, an accident occurs. It is the task of the accident investigation to identify all contributing factors and publish them in a final report such that the holes in the cheese can be filled (meaning the problems can be fixed) so future accidents due to these factor can be prevented.
The 737 MAX crashes were largely the result of the following (for the full list of contributing factors from the Lion Air crash, see the bottom of this answer):

Boeing's decision to only use a single AoA sensor for MCAS and not limit its activation.
A broken AoA sensor on the accident aircraft resulting in incorrect information feeding into MCAS.
Insufficient flight crew training resulting in incorrect response to the runaway stabilizer trim by the pilots.

It is wrong to blame the accidents on a single contributing factor (like only blaming Boeing or only blaming the pilots). All contributing factors are important and the accident reports do not rank these factors by severity.
By now, MCAS has been updated to use both AoA sensors and only activate once:

This required software function operates in unusual flight conditions only and now relies on two sensors, activates only once and never overrides pilots’ ability to control the airplane.

(Boeing)
Most regulators also require pilots to undergo simulator training before being allowed to fly the 737 MAX again:

This is not just about changes to the design of the aircraft: every individual 737 MAX pilot needs to undergo a once-off special training, including simulator training, to ensure that they are fully familiar with the redesigned 737 MAX and trained to handle specific scenarios which may arise in flight.

(EASA)
That way, at least two holes are now filled and even if an AoA sensor breaks on a 737 MAX again, this should not result in another accident.

For completeness, here is the full list of contributing factors of the Lion Air crash. The final report of the Ethiopian crash is not yet published and the interim report does not list contributing factors.

During the design and certification of the Boeing 737-8 (MAX),
assumptions were made about flight crew response to malfunctions which,
even though consistent with current industry guidelines, turned out to be
incorrect.

Based on the incorrect assumptions about flight crew response and an
incomplete review of associated multiple flight deck effects, MCAS’s
reliance on a single sensor was deemed appropriate and met all certification
requirements.

MCAS was designed to rely on a single AOA sensor, making it vulnerable
to erroneous input from that sensor.

The absence of guidance on MCAS or more detailed use of trim in the flight
manuals and in flight crew training, made it more difficult for flight crews to
properly respond to uncommanded MCAS.

The AOA DISAGREE alert was not correctly enabled during Boeing 737-8
(MAX) development. As a result, it did not appear during flight with the
mis-calibrated AOA sensor, could not be documented by the flight crew and
was therefore not available to help maintenance identify the mis-calibrated
AOA sensor.

The replacement AOA sensor that was installed on the accident aircraft had
been mis-calibrated during an earlier repair. This mis-calibration was not
detected during the repair.

The investigation could not determine that the installation test of the AOA
sensor was performed properly. The mis-calibration was not detected.

Lack of documentation in the aircraft flight and maintenance log about the
continuous stick shaker and use of the Runaway Stabilizer NNC meant that
information was not available to the maintenance crew in Jakarta nor was it
available to the accident crew, making it more difficult for each to take the
appropriate actions.

The multiple alerts, repetitive MCAS activations, and distractions related to
numerous ATC communications were not able to be effectively managed.
This was caused by the difficulty of the situation and performance in manual
handling, NNC execution, and flight crew communication, leading to
ineffective CRM application and workload management. These
performances had previously been identified during training and reappeared
during the accident flight.

(Lion Air Flight 610 Final Report)

It is worth noting, while I do agree it is not fair blaming only Boeing, that the MCAS relying only on a single source of AoA is stupid beyond comprehension, and just plain criminally negligent as P Kämpf noted in his answer. The largest hole, about the size of the whole slice, was thus provided by Bojoing... The accident reports do not rank the factors, as it is not their task to establish liability or point fingers, but simply to list causes to prevent similar accidents from happening.

@Jpe61  Particularly since the majority of similar systems rely on not one, but THREE sensors/readings, so that 'voting' can take place - taking readings from two is an improvement, but ONLY if the system has been written to take the correct action when readings differ.

True, and to be precise, a system with high demands on reliability and security, if only two sources of information are available, and they disagree, system should be set in "failed" mode so that system operator can be given the choice of action. With two disagreeing inputs the system cannot usually reliably determine which of the inputs is unreliable.

All the answers here are good, and they address what you were probably intending to ask.  But I want to address the question that you literally asked

Why not more Max 8 accidents?

i.e. why were there exactly two Max 8 accidents, and not, say, three or or five or ten?
Lets review the timeline
October 29, 2018: Crash 1: Lion Air Flight 610
March 10, 2019:   Crash 2: Ethiopian Airlines Flight 302
March 13, 2019:   FAA grounded fleet March 13 (many other countries grounded soon)
Let's imagine a hypothetical parallel universe with this timeline:
October 29, 2018: Crash 1: Lion Air Flight 610
December 1, 2018: Crash 2: XYY airlines flight 123
What would have happened then?  Well, mostly likely the FAA would have grounded the fleet on December 4th, 2018.  And so Ethiopian Airlines Flight 302 would never have happened. You'd still only have two crashes.
You could imagine many different variations on this.  Like maybe Lion Air Flight 610 got lucky, so that Ethiopian Airlines was actually Crash 1, and the some other flight crashed in June 2019 and then the grounding happened a few days after that.  You'd still only have two crashes.
My point is, if there are ever two major fatal crashes within a short time span of each other that appear to be related, the Aviation authorities are going to ground the fleet before there is a third.  Maybe, if the two crashes don't obviously have the same causes, it might get to three. For some cases, the fleet would be grounded after just one crash. But there is no universe in which there are going to be ten or twenty fatal crashes for the same reason without the fleet being grounded.
To be clear, the caveats here are:

Major fatal crashes.  There have definitely been repeated incidents that did not lead to immediate grounding (e.g. fan blade failures like on Southwest Flight 1380), but those did not involve a hull loss and the death of everyone on board
Short time span.  Had the two 737 Max events happened years apart, the second one might not have led to an immediate grounding
Related reason.

Part of this has to do with how the 737-MAX was designed. I explained this in more detail in this answer, but the TL;DR here is

The Max 8 was designed to be as close to identical to existing 737s as possible so that pilot retraining and full FAA certification would not be necessary
MCAS could be shut off using the same methods as a runaway stabilizer (an issue all 737s could encounter). The prior air crew on the craft that was Lion Air 610 figured it out and avoided a crash

The puzzled cockpit crew checked a quick reference handbook, running through other emergency steps before successfully regaining control of the plane by executing the checklist for a runaway stabilizer. That turned off MCAS and the crew flew manually for the rest of the trip.

In other words, it mostly behaved like a well-known aircraft. You just had to be able to diagnose the problem correctly before the plane crashed. It seems, in most cases, the air crew did. The problem is the cost of failing to diagnose it is unacceptably high...


text:
Can we get 0:40 thrust to weight ratio from three small engines
Because I have an engine of 13 hp four stroke 1 cylinder that pulls about 0:20 T/W ratio.  and is it possible to multiply the thrust by two? Using two more small engines which is 6.5 hp each on the both side.

Adding 2 x 6.5hp might double the thrust. What about the extra weight of the engines and required linkages? Looks like this might actually reduce thrust-to-wieght.

How do you get zeros in your thrust to weight ratio?

@jamesqf Easy: just turn the engine off, you get 0 thrust for 40 weight :D

Do you mean a 0.40 and 0.20 thrust to weight ratio? A 0:40 or 0:20 thrust to weight ratio would mean that thrust is zero.

Only if the 6.5 hp motor and propellers have the same thrust efficiency as the 13 hp one.  As the propellers get smaller and spin faster, the efficiency usually drops and if you're getting, say, 50lbs of static thrust from the 13 hp one, you will probably get slightly less than that from two 6.5 hp ones combined.  So instead of 100 lbs from the three, it might be closer to 90-95 lbs.
Then there is the weight of the smaller engines. If the total installation of two small ones is more than the weight of the single larger one, you're getting farther behind in terms of total power to weight.
If the objective is to double the power of an ultralight with an existing single nose mounted motor by adding two small ones on the wings, it might make sense if you can control the weight, but you will probably end up with something significantly less efficient than just replacing the 13 hp motor with a 26 hp one, or putting two 13 hp motors on the wings and removing the one on the nose (assuming all the weight and balance issues can be resolved).

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
How often do talk-down landings go wrong?
There are some well documented stories of non-pilots successfully landing airplanes after the pilot was incapacitated. Usually this involves radio assistance by ATC and/or flight instructors.
My question is whether there is any statistical data available about the success rate of such landings. I would judge success by the lives saved by the landing attempt.
I am aware that the term "non-pilot" is somewhat vague, in that it could refer to anything from a complete novice to a student pilot or even a GA pilot trying to land a commercial jet. Any kind of statistical data involving people punching above their weight trying to land an airplane in an emergency would qualify as an answer.

Interesting question - welcome to Av.SE!

My suspicion is that it would be difficult to get any statistically significant figures on these events because they happen quite rarely. This related question might be a jumping-off point, but of course the newsworthy events will be the ones where the talk-down was successful, otherwise the story would be simply "the plane crashed."

@randomhead That's exactly why I asked. News reports are usually not a good source for statistical data ;) So getting a feeling for whether there are 1, 10 or 100 catastrophic crashes for every successful landing is what I'm looking for.

How does it make any sense to close as "Opinion-Based" a question asking for stats???  Just because it turns out that such stats aren't kept - as far as any of us have found so far - why does the question need to be closed? VTLO, empatically.

@ymb1 Thank you for putting in the effort! I guess "Too rare for a significant figure" is sort of an answer in itself. Would it make sense to post it as an answer for me to accept after a while if no-one else comes along with new insights?

To address the headline of the question - if it’s a talk-down landing then something has likely gone badly wrong already.

@Frog agreed. But as long as the aircraft is still flying, there's still room for things to get much, much worse.

I know of half a dozen successful talkdowns, not always due to incapacity, but more to do with say a student on first solo, losing a wheel on takeoff.
I dont know of any failures.
Stats hard to get, but any assistance is likely to vastly reduce the amount of 'accident'. Even if its steering the aircraft away from populated areas, slowing down, having services on standby etc.

Way better than with no help (call for help).

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is the correct phraseology for changing the runway of a landing clearance?
In the liveATC clip on this YouTube video, an American Airlines 767 arriving at SFO (San Francisco International) had been cleared to land on 28R, but, apparently while already on final, was told to switch to 28L.
The initial clearance given as

American 2293 Heavy, San Francisco Tower, Wind 210 at 18, Caution Wake Turbulence, Heavy 767 mile-and-a-half final, Runway 28 Right, Cleared to Land.

which was read back, as per the usual. However, the Tower later sent the following transmission:

American 2293 Heavy, Change to Cleared to Land Runway 28 Left

which the pilot read back as

28L, Cleared to Land. American 2293 Heavy.

However, the pilot was seemingly too focused on flying the final to notice that the runway assignment had changed between the clearances and continued flying final to 28R, nearly resulting in landing on top of a United jet that was subsequently cleared to line-up-and-wait on 28R. Fortunately, the United pilot noticed the big 767 flying towards him and exited the runway on the other side instead of lining up. Tower then instructed American 2293 to go around.
It seems to me that the phraseology used to change the clearance here is too close to sounding like the controller is just repeating the initial landing clearance, rather than directing a busy pilot's attention to the fact that he was just told to change runways while apparently already flying final.
My question is: Was the phraseology correct for this change of the clearance? If not, what is the proper phraseology?
Given that this was in the U.S., I'm looking for FAA phraseology.

If they're on final I'd like to think they shouldn't change clearance like that! Would it not be better to make them go around anyway or just keep the runway clear so they could land?

I don't know the phraseology and don't have time to look right now but seems like everyone got it wrong.  That amended clearance is nowhere near strong enough.  A runway change on final?  Poor situational awareness from the crew.  How about "cancel clearance" followed by a new clearance and if the crew are unable to do so safely, a go around?

@Notts90 That's my (student pilot, not airline pilot) opinion also, though from a bit of searching around, "side-stepping" seems to be somewhat normal there.

@Simon Yes, my first thought was that "Cancel landing clearance" followed by the new clearance would have been the appropriately-strong phraseology for something as significant as changing an already-issued landing clearance to a different runway, but I wasn't sure what if that was officially 'correct' or not, hence the question.

In my opinion landing clearance cancel and then go-around with lineup for the new runway would have been appropriate.

@BurhanKhalid At KSFO, if you're on final for one runway, you're pretty much on final for the parallel runway too; they're that close. There's a reason for the charted sidestep approach.

The phraseology in the video was slightly incorrect at the time it was issued. Due to further clarifications, that phraseology is now even more of a departure from the current guidance.
When that video was posted in April 2015, the current version of the ATC Order was FAA JO 7110.65V, Change 2. Paragraph 3–10–5 prescribes the "change runway" phraseology, and that paragraph was not modified in Change 2 nor in Change 1; thus the paragraph published in JO 7110.65V "basic" was in effect:

3−10−5. LANDING CLEARANCE
a. When issuing a clearance to land, first state the runway number followed by the landing clearance. If the landing runway is changed, controllers must preface the landing clearance with “Change to runway.”
PHRASEOLOGY−
RUNWAY (number) CLEARED TO LAND.
Or
CHANGE TO RUNWAY (number) CLEARED TO LAND

That means the ATC instruction should have been:

American 2293 Heavy, change to runway 28L, cleared to land

All the correct information was in the transmission you quoted, it was just given slightly out of order. But that could easily be an issue for a pilot who's in a high-workload phase of flight and it's probably exactly why the ATC orders say that the words "change to runway" must come first.
Paragraph 3–10–5 was modified by Notice N JO 7110.761 dated October 26, 2018:

3-10-5. LANDING CLEARANCE
a. When issuing a clearance to land, first state the runway number followed by landing clearance. If the landing runway is changed, controllers must preface the landing clearance with “Change to runway” followed by the runway number. Controllers must then restate the runway number followed by the landing clearance.
PHRASEOLOGY-
RUNWAY (number) CLEARED TO LAND
Or
CHANGE TO RUNWAY (number), RUNWAY (number) CLEARED TO LAND.
NOTE-
The purpose of the ‘change to runway’ phraseology and restating the runway number is to emphasize to the pilot that they are being cleared to land on a runway other than what they were expecting.

The background information in the change notice says that "This new language builds on previous mitigations and is intended to clarify the "runway" on which a pilot is to land."
N JO 7110.761 was incorporated into JO 7110.65X, Change 3, dated February 28, 2019.

I've had ATC change my landing clearance a couple of times but the controller always asked me first if I could do it:

Tower: N12345, cleared to land 21L
N12345: Cleared to land 21L, N12345
[A few seconds later...]
Tower: N12345, are you able to switch to 21R?
N12345: Affirm, we can do that, N12345
Tower: N12345, change to runway 21R, cleared to land

However, landing a C182 at a Class D is a bit different from landing a heavy at SFO; I assume the controllers there have less time to talk so they may simply have changed the clearance and expected the pilot to say "unable" if necessary.

Thanks for the comment on my answer. Its been a while since I had to read up on US R/T. Your answer is now, technically, more "correct" than mine as the OP asked specifically for US procedures. I think i'll leave mine as its a "general" answer.

This has happened to me twice. Once the clearance was "cancel landing clearance, now cleared to land runway XX". The other time it was "correction, runway change, cleared to land runway XX".

Was the phraseology correct for this change of the clearance?

Probably not. It was possibly missing an important standard phrase, and almost certainly missing the clarity that such a change should include. There was also some implied diligence, on the part of ATC, to ensure that the change in clearance was understood by the pilot.

If not, what is the proper phraseology?

There is a missing Phrase in the ATC transmission, and both the UK CAA CAP 413 and the ICAO Radio telephony Manuals both agree (I will assume the FAA do too, as they tend to follow suit, but for my google-fu I cannot find the official source):

CORRECTION - An error has been made in this transmission (or
  message indicated). The correct version is...

So, the ATC's instruction should probably have been along the lines of:
American 2293 Heavy, CORRECTION to clearance to land. Cleared to Land Runway 28 Left.
To which the aircraft should have responded
28 Left, Cleared to land, American 2293 Heavy
Should the controller have not received the correct read back of the assigned new runway...
28 Right, Cleared to land, American 2293 Heavy
The controller's job, would have been to correct that mistake immediately
American 2293 NEGATIVE, Cleared to land Runway 28 LEFT. Confirm runway change received.
To which you would hope the pilot got the point that his clearance has changed, and he would read back the corrected landing clearance.

There are 2 other possibilities. My opinion is that the above would be slightly more correct but for answer completeness:
The controller could have cancelled the previous clearance using that standard phrase CANCEL [previous clearance] and reissued it.
American 2293 CANCEL clearance to land Runway 28 Right. Cleared to land Runway 28 Left
To which an accurate readback would have been expected, with the same diligence by ATC to correct any mistake. At least in this case it is clear from the message that the landing clearance has changed.
The other option, and its a real outsider and less standard (as its intention is to reclear for the purpose of routing) is the standard phrase RECLEARED
American 2293 RECLEARED to land Runway 28 Left
Again, this should have made it clear to the pilot that his clearance has changed, and the same readback/check cycle would be initiated. I think this particular example is moving away from standard phrasology, and I only include it for completeness.

As you can see, even standard phraseology is sometimes ambiguous as to the correct one to use. It's why controllers & pilots have some leeway (we're humans communicating at the end of the day). The important part here that was missed was an accurate message/readback/confirmation cycle.

Does anyone know if the US just follows ICAO for RT phraseology? There is no RT phraseology guide on the faa website that I can find.

The main sources in the US are the P/CG and ATC Orders. Other sources like the PHAK have information too, but I'm not aware of a single FAA RT guide; the P/CG is probably the closest thing, and it does highlight differences between US and ICAO phraseology.

This answer provides useful information and I upvoted, but I don't think 'correction' would really have been appropriate here since the initial clearance wasn't read incorrectly (as far as I can tell, at least.) It appears that the initial clearance was intended as given and that it was later changed to a different clearance for some operational reason.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why is there only a set amount of shockwaves when supersonic flow passes a curved surface?
In the picture below, why are there only 4 shockwaves created on the surface? Firstly, I'll label the shockwaves one to four.
So, the number 1 shockwave is the one furthest forward, and the number 4 shockwave is the one furthest back, so the last one.
If you look closely in between shock 1 and 2, the surface curves a small amount upward. Why, in this space between 1 and 2, is there no shockwave? Shouldn't there be an infinite amount of shockwaves there because every micrometer that the flow moves to the right the surface curves up by a small amount (presumably creating a shockwave)?

I'm assuming that all shockwaves in this picture are oblique shock waves, but I could be wrong.

I think you are misinterpreting the graphic...  Because why are there only three blue airflow arrows?

@MichaelHall That is a good point. Maybe it is just trying to give the viewer a general idea? If that is the case, it is kind of misleading.

Are the blue arrows misleading?  Do they make you think that there are only three streamlines?  Just because we misinterpret a figure does not mean it was misleading.  Sometimes it is our opportunity to learn.  More importantly, you are missing the point of the figure -- those are not shock waves (abrupt jump in compression).  Instead, those are 'Mach waves', a gentle gradual alternative to shock waves.

@RobMcDonald Yeah, maybe I was just misinterpreting it. So, if the flow is M > 1, then why aren't those shockwaves? Supersonic flow and a surface that turns should make a shockwave, right? My theory is that because maybe the amount of flow turning needed is so slight that only Mach waves need to form.

That is essentially correct.  Mach waves occur when a very small disturbance occurs or a very slight amount of turning.  In reality, we see this occur when there is some roughness on a surface, when a surface has a very gradual curved ramp, or (most commonly) due to shock-boundary layer interaction -- where the boundary layer acts like a somewhat soft boundary, causing the flow to turn gradually.


text:
Identifying the behavior of these two planes
I have no aviation knowledge/experience but I observed something on a flight tracking app I was curious about:
Early morning (7 AM) I watched two small planes (one of them was a piper archer) traveling close together (visually: they stayed a few streets apart) around 100 kts at an altitude of 1200 feet with both planes staying at minimum 300 feet apart in altitude.  They traveled together for about 30 minutes while one plane landed and the second one flew to another airport.  The second plane eventually landed at the airport but almost immediately took off after coming to a stop (say 20 seconds of time on the ground).  It did a loop around the airport and then landed and didn't take off again.
My question is: what explains the behavior of the two planes?  Was this some sort of training exercise?  I've never seen two small planes travel so closely for so long so I'm curious.

Perhaps you have noted the registration number in the tracking application? This would allow to check the business of the owner.

The planes travelling together may simply be a coincidence. They had matching flight plans till the first one landed, and were simply maintaining visual separation as per Visual Flight Rules (VFR). It may also have been a formation flying exercise, but the planes landing at separate airports does not support this assumption.
The second plane seems to have made a stop-and-go landing, which is a training procedure where you land the plane, come to a full stop and then proceed to make a takeoff. If the runway is sufficiently long, the takeoff may be made without taxiing or backtracking to the beginning of the runway.

They might have been doing a photo shoot.


text:
How stable are 3+2 seat aeroplanes (A220)?
I imagine they should steer to the right, as more passengers sit on that side? Are these planes manufactured differently (non symmetrical wings)?

Related

The effect of the passengers on balance is much less than the effect of fuel load. An A220 can carry around 17 metric tons of fuel, which weighs more than 150 average passengers and is at a much higher lever arm than the passengers.
Any imbalance would cause the plane to roll slightly if the ailerons were kept perfectly neutral. But this is easily counteracted by a little aileron trim.


text:
Minimum HP for ultralight
What is the minimum HP needed to fly a 200 pound ultralight? I’m looking at a 6.5 HP, 3600 RPM Predator engine, but is that enough?

Please see the following questions and their answers: https://aviation.stackexchange.com/q/72021/7532 https://aviation.stackexchange.com/q/33476/7532 https://aviation.stackexchange.com/q/70432/7532 https://aviation.stackexchange.com/q/45160/7532 https://aviation.stackexchange.com/q/74586/7532 https://aviation.stackexchange.com/q/68105/7532

That would be a little sketchy, given that commercial ultralights have motors ranging from 15 HP to 120. I’d opt for something within that range. Sure, it might be possible, but you would need an extremely light, aerodynamic craft for that to be a legitimate plane, instead of hovering a few feet off the ground. 15 HP motors would be a better option.


text:
Do airliners respond to mild turbulence in real time?
I've always been fascinated with the impact of ailerons on an aircraft's trajectory; from what I have seen sitting by a wing is that the slightest movement can bank the aircraft quite significantly.
On my last two flights we had mild to moderate turbulence and I noticed that nearly every movement appeared to be counteracted. That is, if the plane moved to the left, it was (in second to sub-second time) accompanied by a sensation of moving to the right. It's the same for up and down.
What I couldn't decide and what I can't seem to research properly is between these options:

Given the small adjustments needed on flaps, and the speed of software and hydraulics these days, the aircraft is actively correcting for every small disturbance it can, similar to traction control in cars
The aircraft design is just self-righting, similar to a gyroscope. The fact that a left movement seems to be accompanied by a right movement is just the fact that the force pushing the aircraft left was fleeting and the aircraft just abruptly stops being forced left, giving me the sensation that I then pulled to the right
I'm a human being trying to find patterns in something and, actually, the plane is just being buffeted randomly

Is the aircraft actively responding in real time to these minor disturbances?

My main curiosity is just whether a commercial airliner is actually making adjustments in fractions of a second in mild turbulence or whether it was just the property of the plane design itself (point 2). I perhaps presented it incorrectly in the question. The answer seems to suggest that indeed the aircraft really is as responsive as what I thought I felt

Stability by design
Transport aircraft are inherently stable. They are designed with a positive static stability, meaning they tend to automatically correct for variations in their attitude and return to neutral. E.g., the wings are not built horizontal; they make an angle (dihedral or anhedral) to return to zero bank after a wind gust. Without this roll stability, the aircraft could continue to roll after this motion has started.
However, this stability mustn't make piloting difficult, e.g., too much roll stability would require excessively large efforts on control surfaces. The compromise is a design choice, e.g., fighter aircraft have less stability in order to facilitate fast maneuvers, while transport aircraft are less agile, but more stable.
With less stability, safe piloting may require computers (e.g. to move canards). The same principle of aided flight is also used on more stable aircraft, to relieve the pilot from constantly adjusting attitude, or to react faster than the pilot, e.g. for gust compensation.
When you feel instantaneous reaction, it's likely an action of these computers.
Flight computers for additional stability and protection
Additional stability isn't natural, but managed by computers distinct from the autopilot maintaining programmed route and altitude, dedicated to the actuation of control surfaces. This system is what is generally called fly by wire.
Computers receive pilot orders, calculate a combination of surface movements, within predefined safe limits, send electrical signals to actuators, ensure the calculated changes are carried out, then maintain the commanded attitude.
For attitude keeping, the principle is to detect accelerations and orientation changes on each of the three axes. This is usually done by the inertial reference system which also computes the aircraft position. These inertial data are used by the flight computers.
One example of such computer is the yaw damper, which counteracts both a yaw and roll oscillation in order to prevent a flight dynamic mode known as Dutch roll which can be catastrophic if left uncontrolled, and has been the cause of accidents.
Airbus example
On Airbus aircraft, attitude correction is controlled by seven computers (ELAC, SEC, and FAC, see details in Airbus control architecture - where does the actual autopilot live?) which can reconfigure themselves if one or more fail, to continue providing protection and stability as much as possible. They work regardless of who pilots, human or autopilot.

These computers set the attitude commanded by the pilot, and maintain it when input has ceased. They also ensure the commanded attitude and the commanded rate of change are within the possibilities of the aircraft (flight envelope protection). On the schematics above there are two mechanical links which are last resort manual controls to land the aircraft in case the fly by wire system has failed to the point orders cannot be transmitted.
For bank stability, this is the way Airbus A320 family works. From manufacturer FCOM:

and:

When the autopilot is engaged, which is typical during cruise flight, it's working to hold the wings level and maintain the desired pitch most of the time (a programmed turn or climb/descent to follow the navigation being the exception). So it detects any deviation from wings-level or desired pitch and makes a control input to promptly return to the desired attitude.
You see that control input with the aileron and spoiler deflection; you don't see the elevator inputs for pitch control since that's behind the cabin.
A human pilot flying the airplane does much the same thing, perhaps not quite as quickly as the autopilot.
If neither the autopilot nor the human pilot were providing the corrective inputs, then you wouldn't see the ailerons moving, and the aircraft would spend far longer in an attitude other than wings-level, and probably wouldn't hold altitude very well.

Can the autopilot really counteract the motion of turbulence within a second? That's what blows my mind if theory (1) is true.

Within one second? Sure. Consider driving on the highway in gusty winds... you react way faster than 1 second when a gust starts to push the car out of the lane, and digital autopilots can recognize an acceleration in the roll axis even faster than we can perceive it.

I love the way you say that so casually when we're talking about several hundred tonnes being moved around without the traction that a road would give you in the car analogy! :)

Fair point... the car analogy is more about how fast the roll can be perceived; the sensitivity of the controls is a different topic. Airliner flight controls are not as sensitive as those of acrobatic aircraft, where a little push can send you rolling over & over! But, they do have ample authority to do all that's needed to keep the airplane pointing where it needs to go, and the autopilot is tuned to command the right amount of deflection - powered by 3,000 psi hydraulic systems - to arrest the undesired roll & return to level flight promptly & smoothly.

The question mentions left/right and up/down movements.  I suspect the autopilot is controlling roll, pitch, and yaw.  These are not the same things.  I am guessing that people do not even notice the roll/pitch/yaw caused by turbulence.  Could you add some more detail?

@AnonymousPhysicist Added comments for pitch, though the same basic principles apply. Light to moderate turbulence doesn't tend to induce very much yaw, at least with a yaw damper engaged (which it is full-time, if everything is working as normal).

@roganjosh Maybe it helps to think thus: the airplane has a wing moving through the air fast enough to not only produce the several hundred tonnes of lift needed to keep it there, but also, at cruising speed, cause structural failure if the control surfaces are grossly mishandled. Also, the only reason turbulence can shake the airplane in the first place is through aerodynamic forces on the airfoils. On the other hand, in a car, we put our trust in a few square cm. of rubber not slipping...

I suspect you were flying on an aircraft with a gust/load alleviation system such as the Boeing 787, Airbus A350 or A380.
In these aircraft, accelerations due to turbulence are measured in the wing and immediately counteracted by deflecting the control surfaces.
This works independently from the autopilot.
It sometimes gives the impression that the controls are somewhat randomly fluttering in the wind.
From this answer:

A gust alleviation system is a control system fitted to some
Fly-by-wire (FBW) aircraft that reduces the effect of gust loads on
the aircraft by deflecting control surfaces such as ailerons, rudder
and elevators.
The system works by measuring the upward acceleration of the aircraft
and comparing that with the acceleration commanded by the (auto)pilot.
A feedback loop adds a correction signal to the signals controlling
the deflection of the control surfaces in order to counteract the
accelerations cause by wind gusts.
In its simplest form the accelerations are sensed near the center of
gravity of the aircraft. More advanced implementations work with
multiple sensors in the body and in the wings. These gust alleviation
systems do not only attempt to annul the effect of the gusts on the
aircraft's normal (upward) acceleration, they also reduce the wing
bending moments. This in turn reduces metal fatigue.
In the A380 the load alleviation system is nicknamed "valse des
aileron" (Waltz of the Ailerons) because of the dancing movements of
the aileron:

See also here

Since the wing is elastic "measured in the wing" probably means the response occurs well before the passenger gets shaken.  I suspect these gust alleviation systems work so quickly that they do not contribute to the passenger's feeling of a "response."  I think they just reduce the amount of movement the passenger feels.  Do you have any more information?

@AnonymousPhysicist they certainly reduce the amount of movement the passengers feel, but there are still residual forces transferred from the wing to the fuselage. So while the start of the response may take place before the elastic wave through the wing has reached the fuselage (but this is not necessarily the case), the wave head will reach the fuselage. The GAS reduces the magnitude of the wave.

Gusts can also be detected at the nose, before they reach the wing. At 500 kt this would leave about 0.1 s for the actuators to move ailerons and spoilers, when gusts are ahead.

Most Airbus models have a turbulence damping function. The early A320 had it but it had technical issues and was dropped. The sharklet models got it back due to wing loading. A330/A340 had it due to fuselage length, it dampened flex modes for fatigue life and comfort, it was a fundamental benefit of FBW.

That is, if the plane moved to the left, it was (in second to sub-second time) accompanied by a sensation of moving to the right. It's the same for up and down.

Human perception is a bit strange.  When you feel the plane move "up and down," you are not feeling its position.  You are feeling acceleration.  If:

The plane is flying at constant altitude.
You hit some turbulence and feel and "upward movement", then the plane is accelerating up.
The plane's altitude increases.
Very soon, you feel an equal "downward movement", then the plane is decelerating up.
If the two accelerations are equal (but opposite), then the plane is flying at a constant altitude, but now it is higher up.

Yes, the aircraft autopilot will return the aircraft to the selected altitude.  However, that's probably not what you are feeling.

I'm a human being trying to find patterns in something and, actually, the plane is just being buffeted randomly

That is how humans normally work.  If you take a bunch of random buffets, most of them are going to cancel out quickly.
Airplanes (and particularly large modern airliners) are not rigid.  You need to consider the bending of the aircraft as well as the buffeting to understand the passenger experience.

https://efcms.engr.utk.edu/ef151-2019-01/m1/pilot/class15/SVA%20diagram.png

Given the other answers, I'm not convinced this was an issue of perception. The fact that I was aware of my own fallibility at the time when I was trying to tell whether there really was an effect, and a background in engineering, I tried really hard to remove that bias. I don't recall feeling "corrective force" in flights I've had in the past so this felt quite distinct as direct opposition to buffeting. Of course, it's hard to tell when turbulence is... turbulent and you don't get clean 1:1 signals since it's happening almost continuously

@roganjosh If you are saying you observed different behavior on certain flights, then I don't think my answer will help you.  Maybe it was a different type of aircraft.  Maybe it was different turbulence.

Do airliners respond to mild turbulence in real time?

More or less.
Gust alleviation is a very important aspect in aircraft design. It is so important that big companies have a whole department only devoted to gust studies. For a jetliner entering a gust, the relevant loads are indeed the most critical loads normally encountered in its life and therefore its structure is designed around those loads. A reduction of those loads leads to savings in weight and fuel as high as 5 to 10%.
The following picture (source) shows a typical distribution of gust loads on a jetliner wing:

Especially important is the increase of the aerodynamic loads towards the tips of the wing since it generates a high bending moment at the wing roots.

There are many methods to counteract those gust loads but they can be classified either as  passive or active according to their working principle.
A passive method can be for example to design the structure of the wing in such a way that when it bends upwards, the local AoA gets reduced reducing the local aerodynamic loads and therefore the bending moment.
A standard active method implies instead the use of gust sensors feed in an automatic system which uses some control surfaces to counteract the load increase. In this case, ailerons are typically used since they are already located at the wing tips i.e. where the highest gust loads happen. Using ailerons, spoilers or flaps is convenient also because they are already there i.e. there's no need to add ad-hoc surfaces with their relevant weights. Anyway, as you suspected, the whole loop (from when the sensors pick up a gust till the ailerons get moved) is not as fast as the whole gust spectrum, which ranges from few to hundreds of oscillations per second. Being ailerons, spoilers and flaps mainly sized for manoeuvring and high-lift devices, they are large and heavy and hydraulically actuated. They are therefore quite slow and virtually ineffective against the high-frequency part of the gust.
To gain some precious milliseconds, Airbus tested some time ago a "high-resolution direct-measuring short-pulse ultraviolet (UV) Doppler lidar system". This system detected gust movements till some 50m in front of the airplane giving 300ms lead time to the gust control system. I don't know if this particular system made it out of the experimental phase.

That thesis is bang on the nose as I'm reading through the introduction. Thank you for adding that

@roganjosh: you're welcome


text:
How does AWACS pick up enemy info like speed and altitude without transponder interrogation?
So basically, IFF can pick up friendly vehicles. I'm assuming that enemy planes will have their transponder off to minimize speed and altitude data. How does the AWACS determine enemy info to pass on to friendly fighters?

Do you know what that giant disc on top the airplane is?  (not being snarky, just trying to establish a knowledge baseline...)

@MichaelHall yes that's the radar with the IFF interrogator correct?

That disc does much more than just IFF.

I think the IFF is a separate function, but you have a good answer now so never mind.

And the E-3 AWACS is being replaced by the 737-700 based Wedgetail. https://www.defensenews.com/air/2022/04/26/its-the-wedgetail-air-force-to-buy-e-7-to-replace-awacs/

-1 for not knowing what radar is. I wish I could use more minuses for actually using the word "radar" without knowing what radar is. (I'll admit to snark, but this is special.) Look up "radar" on Wikipedia, or even just in the dictionary, and think about how your question needs updating.

How does a cop know how fast you're going in your car using a radar gun when your car has no IFF transponder?

Re, "enemy planes will have their transponder off..." If any aircraft in the air is an "enemy," then every aircraft is somebody's enemy.

Primary radar doesn't require a transponder or IFF module. It works by actually bouncing a radio signal off the skin of the airplane. It doesn't require the cooperation of the plane being detected, so it works even for enemy aircraft. The direction the antenna is pointed when the signal is received tells you the direction to the target, and the delay between transmission and reception tells you the distance.
Altitude can be determined the same way as position. If you know the target's elevation (vertical angle with respect to the horizon) and distance, it's a matter of simple trigonometry to find its altitude.
(Civilian radars normally don't tell you the elevation, because making the radar antenna go up and down in addition to in a circle was considered too much of a technical challenge for early radars, and transponders came along before vertically-scanning radars. But for military use, where the target probably isn't going to tell you their altitude just because you asked nicely, vertically-scanning radars are common.)
(Also note that modern radars can be aimed via solid-state techniques, so just because you don't see the antenna moving vertically doesn't mean it isn't scanning vertically).
Speed is determined simply by tracking how far the target moves between successive sweeps.

Isn't speed determined by the strength of the doppler effect on the returning signal? This means you can get the speed within a single sweep.

@LordOfThePigs - that would be the relative speed, surely?

@MichaelHarvey Right, but getting the current ground speed from the avionics or GPS and adding it to the Doppler result shouldn't be too hard.

@LordOfThePigs Are you going to get a doppler shift if the target's flight vector is perpendicular to the vector from the radar station to the target at the time it gets pinged?  I'm not convinced that a single ping will be sufficient to get a velocity in all cases.

Also note that military radar showing the (potentially less precise) radar altitude instead of the one broadcast by the transponder can be misleading. This may have been a contributing factor to the 2006 midair collision of a business jet and a Boeing 737 (https://en.wikipedia.org/wiki/Gol_Transportes_A%C3%A9reos_Flight_1907) - the business jet was at FL370 (on a collision course) the whole time, but the radar altitude fluctuated and the controller didn't notice that what he was seeing was not the "transponder altitude". If he had just seen a dot without any info, it would have been clear.

@LordOfThePigs Doppler shift only tells you the velocity component that's in the direction of the radar antenna. The component that's perpendicular to that line wouldn't be detectable with just Doppler alone.

Taking away the doppler is why the first step in defending against a target tracking radar is to turn and put the threat off the left or right wing.

Range, Azimuth, and Elevation and rate of change of those parameters, along with Doppler are all used by the tracker to establish target speed and heading. And they are all cross checked to detect countermeasures. Airborne self-protection jammers (ASPJ) can spoof radar returns altering range and Doppler measurements to throw the tracker off. Of course the tracker has counter-counter measures to attempt to negate the counter-measures. It's a never-ending game. I was testing CCM software when I was in the USAF 35 years ago.


text:
What does 15/13 mean with pilot schedule?
Saw that this when looking at a part 135 corporate pilot job. What does this mean?

That generally refers to days on / days off.  In this example you could expect 15 days on duty, followed by 13 days off.


text:
If ADS-B is on majority of aircraft, why don't we start seeing ASR dying out?
Are radars still needed, or will radars start coming down soon?

I don't believe the military would agree to an aircraft having the ability to fly through US airspace undetected by merely turning off the ADS-B.

ADSB only works on aircraft that want to be seen.   Radar continues to identify targets ranging from Birds to Balloons to non-compliant / malicious drones and even potentially threatening aircraft.

There are still wide areas where ADS-B is not required and many aircraft without it. If an aircraft will only be flying in Class D, E, and G airspace below 10,000 feet, most of that airspace does not require ADS-B. Even in airspace where ADS-B is required, it's possible to get exemptions for non-routine flights without ADS-B. Aircraft without electrical systems are also exempted from some of those requirements.
These are basically the same requirements for operating with a Mode C transponder. But without a basic transponder, at least ATC can still get a primary radar return for a location. If ATC could only rely on ADS-B, they wouldn't get any location for those aircraft. It's probably not the best solution to just accept that these aircraft would be invisible to ATC.
And even aircraft with ADS-B can have systems fail, either ADS-B directly or something else that requires ADS-B to be turned off or lose input like position. It's also possible that the ATC side of the ADS-B system could fail. In these situations it's very helpful to have another system to fall back on.
Maybe you could argue that in areas where ADS-B is required the radars are no longer needed. But those are also the areas where it would be most helpful to have a backup method of locating aircraft. They are also still providing radar services outside the areas where ADS-B is required.
In order to get rid of radars, new rules would have to be enforced that require ADS-B in much more airspace.

Radars are still needed. Aircraft have failures. Pilots accidentally turn off transponders. Radars have weather modes and thunderstorms don't have transponders. Even flocks of birds can show up on radar. Finally, current rules allow flying in certain airspace without even having an electrical system onboard.
ADS-B is an enhancement providing more data and faster refresh, but it is not a replacement.

On the ground, weather radar is different from primary surveillance radar, so there's no effect when eliminating one or the other. Weather radar is 3D instead of 2D, runs in a doppler mode, and senses much weaker returns, so it's much slower (6 minutes/scan on TDWR versus 1 second/scan on PRM), and some (TDWR) operate on different frequencies. Use of phased array radars to make one radar that can scan weather in between aircraft is in a research phase, but until that happens, weather radar and PSR are completely separate.

It sounds like these problems could be solved by making ADS-B mandatory in all airspace for all aircraft, prohibiting avionics from failing and pilots from making mistakes, and making storms and birds carry transponders!


text:
Why would older jets and turbo props use electric gyros vs vacuum driven?
What is the reason for older jets and turbo props choosing electric driven gyros vs vacuum driven gyros?

Electric gyroscopic systems provide redundancy to vacuum powered instruments.
"Older" (and modern) jets and turboprops are required to fly in all kinds of weather, including iceing conditions which may interfere with some vacuum driven systems.
The likelihood of complete electrical failure in a multi-engine aircraft is extremely low, and electric instruments can still function for some time on batteries.
Interestingly, vacuum systems came along first, some driven by simple venturi tunes on the outside of the plane, with only the magnetos providing spark for the engine.

More reading here.


text:
Are ex military airplanes experimental?
What makes a plane experimental, and why do all warbirds categorize as experimental aircraft?

Where did you find the info that all warbirds are categorized as experimental?

Does this need an 'FAA-Regulations' tag?

"Experimental" refers to a particular category of airworthiness certification available for an aircraft. 14 CFR §91.203 requires all "civil aircraft" to have an airworthiness certificate, basically a piece of paper saying "someone has convinced the civil aviation authority that this particular plane is fit to fly."
There are two broad groups of certification categories: "standard" and "special." Your typical GA aircraft or airliner has a "standard" airworthiness certificate, which is based on the manufacturer's type certification. The manufacturer (say, Cessna) spent millions of dollars to get this type certification, and that makes it easy for Cessna owners to get a standard airworthiness certificate as long as their aircraft conforms to the specifications in the type certificate.
But of course a military aircraft doesn't necessarily have a civil type certification, because in its intended role, it didn't need one. Neither the manufacturer nor a private owner has much interest in spending the millions of dollars needed for a full type certification. So instead of applying for a standard airworthiness certificate, the owner looks at the various options for special airworthiness certification. A warbird isn't a "light sport" aircraft, or an ultralight, or a cropduster, etc. which rules out most of the available categories.
That leaves...

the experimental category:

A special airworthiness certificate in the experimental category is issued to operate an aircraft that does not have a type certificate or does not conform to its type certificate and is in a condition for safe operation.

and, for types that do in fact have a civil type certificate, the limited category:

A limited category special airworthiness certificate is issued to operate surplus military aircraft that have been converted to civilian use under the following conditions:
...

The aircraft has a limited type certificate.

Some warbirds, like the P-40 Warhawk, do have a limited type certificate and can get limited airworthiness certificates.
* The links and citations here are US/FAA-centric, but many (most?) countries' civil authorities have very similar certification systems.

You missed the "restricted" category where the aircraft is intended for specific, special operations. This is the most common category for military aircraft holding civil certification. The C-130/L-100, A400M, and P-3 are all examples holding a restricted TC. The key difference is a limited category aircraft can't be used for hire.


text:
Is there a reason for the difference in turbo prop exhaust placement?
The TBM 940 and Cessna caravan both have the PT6A turbo prop engine. The sister caravan has one single exhaust stack under the engine cowling, and almost all other small turbo props have dual exhaust on both sides of the engine cowling. Is there a reason for this and is one better than the other?

Your picture of the Caravan does not have a PT6A. Aftermarket companies convert them to the TPE331 which has a rear exhaust outlet, thus the location of the exhaust pipe. If you had references for your pictures this would be easy to verify. see for example

Sorry about that, I was just trying to show how the caravan has the exhaust under the cowling regardless of the engine.

The C208 photo DOES have a PT6A. The exhaust exits much closer to the front than the TPE331 exhaust. Also, your reply does not answer the question. The reference for the C208 photo is https://commons.wikimedia.org/wiki/File:FedEx_Cessna_208B_Grand_Caravan_exhaust.JPG FedEx does not operate TPE331 C208.

Please include image sources where possible.

This depends on whether the engine has its exhaust outlet in the front of the engine or the back, and whether the fuselage is pressurized or has cabin air inlet vents. You'll put the exhaust beneath the fuselage to keep it from blowing fumes into the cabin air inlets unless the fuse is pressurized, in which case the cabin air is bled off the compressor and exhaust contamination can't occur.


text:
Does Boeing use a similar display to Airbus ECAM to view a particular system?
Airbus has a dedicated ECAM panel forward of the throttles which is used to select a system for display on the LCD. Is there a similar panel used to select a particular system for display on Boeing's EICAS?


text:
Is there any public available documentation about Honeywell FHDB files?
I am new to aviation, my task is to read FHDB (Honeywell Fault History Data Base) files.
They seem to be kind of tab separated values but with special bytes at line start.
like: 0D 0A A7 09
Some lines in some files begin with # after the mentioned bytes.
On the other hand i know the meaning of certain columns like flight leg or flight phase.
I am more interested in the technical aspects on reading and decoding such files (are there other files that use same format?)
I would appreciate any public info or tools (i only found this https://apps.apple.com/vn/app/honeywell-forge-mxadvisor/id1002600365) that help with this task. For reading i will use Python.

0d 0a is a Windows-style line break. 09 is a tab.

Hi Welcome to Aviation. Unfortunately resource location questions are off topic here.


text:
Difference between velocity potential and stream function?
I am currenty reading Fundamentals of aerodynamics(sixth edition) Jhon Anderson and in sections 2.14, 2.15 was introduced stream function and velocity potential concepts. I understand that both of them describe velocity vector fields, but I cannot fully distinguish difference between them. What is the difference in geometrical interpretation, where one approach can be more suitable than another, and why?

Stream function $\psi$ lines end up parallel to streamlines.  Whereas velocity potential $\phi$ lines are perpendicular to those.  In many ways they are opposites - but think of them as complimentary.
A more mathematical explanation (too mathy for Anderson) starts with the Helmholtz Decomposition (aka the fundamental theorem of vector calculus).
Helmholtz's theorem says that any vector field that is sufficiently smooth and that dies off at infinity can be expressed as the sum of two parts.
$\vec{V}=\nabla\,\phi+\nabla\times\vec{\psi}$
The first part $\nabla\,\phi$ is irrotational.  I.e. the curl ($\nabla \times \nabla \phi$) will always be zero.  There is no $\phi$ that can make this not true.
The second part $\nabla\times\vec{\psi}$ is selenoidal.  I.e. the divergence  $\nabla \cdot \left(\nabla\times\vec{\psi}\right)$ will always be zero.  There is no $\psi$ that can make this not true.
When we think of this in terms of fluid dynamics, we apply Helmholtz's decomposition to the velocity field $\vec{V}$.  When we think about inviscid, incompressible flow, that is a flow that is both irrotational and selenoidal.
You probably recognize that an irrotational flow is one where the vorticity is zero everywhere.  The conservation of mass for an incompressible flow is equivalent to stating that the vector field is selenoidal (we don't usually use this word in fluid dynamics).
So, both of the conditions that apply to each of the terms in the decomposition apply to the flow that we're interested in.
What is a vector decomposition anyway?  We're separating parts.  If we had a general vector field (one with both curl and divergence), then the HVD would break that field into two -- one that represents all the curl and the other that represents all the divergence.  In our world, we're going to want to use these as unknowns that we're going to want to solve for.
Note that $\phi$ is a scalar, even when $\vec{V}$ is 3D.  $\phi$ can be called the potential, the scalar potential, or the velocity potential.
However, $\vec{\psi}$ is a vector in 3D -- but it simplifies to a scalar when we're only interested in 2D problems.  $\vec{\psi}$ is called the vector potential.  In 2D fluids problems, $\psi$ is called the stream function.
When we're solving problems, we need unknowns and we need conditions or equations to apply them to.  We have two choices of approaches.

We choose to apply the continuity equation.  I.e. $\nabla \cdot \vec{V}=0$.  We immediately know that the second term in the HVD can not contribute to the divergence, and so we can drop that term.  This leaves us with

$\vec{V}=\nabla\,\phi$
Which means that $\phi$ is our unknown.  We proceed to study potential theory aerodynamics with the continuity equation and the velocity potential as our unknown.

We choose to apply the vorticity equation (irrotational flow).  I.e. $\nabla \times \vec{V}=\vec{0}$.  We immediately know that the first term in the HVD can not contribute to the curl, and so we can drop that term.  This leaves us with

$\vec{V}=\nabla\times\vec{\psi}$
Which means that $\vec{\psi}$ is our unknown.  We proceed to study aerodynamics with the vorticity equation and the vector potential as our unknown.
In 2D, approaches 1 and 2 are extremely similar and either can be used.
However, in 3D, our unknown in approach 1 ($\phi$) remains a scalar, where our unknown in approach 2 ($\vec{\phi}$) is now a vector (i.e. it has three components).
The vector nature of the second approach makes it substantially more cumbersome in practice.  Consequently, we seldom deal with the second approach after initially introducing it at this stage of teaching aerodynamics.
The first approach goes on to be one of the most important tools in aerodynamics.

I cannot get If in approach 2 we assume that flow is irrotational(curl V = 0) why V consist of ψ which describe rotational part of flow? In section 2.15 of book from irrotational equation (curl V = 0) was derived V =∇ϕ contrast to your V =∇×ψ, why?

The derivation shown in 2.15 is more like a derivation of the first term of HVD than it is my approach 2 (which is a conclusion from HVD).  I did not derive HVD at all.  Anderson tries to show that when $\phi$ is a scalar, then $\nabla \phi$ will be irrotational always.  I just flipped through the book.  Anderson never talks about the second term of the HVD, never uses the term HVD, and never talks about my approach 2.  So, from Anderson's point of view, $\psi$ is some other field that exists and generates streamlines.

Is it proper to say that ∇ϕ describes the irrotational part of the flow and ∇×ψ describes the incompressible part of the flow? If so, in approach 2, we assume that V is irrotational shouldn't we set ∇×ψ to 0(so far ψ contains vortex information) in this case?

@AlexGenesis I think you have it a bit backwards.  A general flow can be compressible and rotational (and can be described by HVD). In approach 1 Since $\nabla \phi$ is incapable of describing a rotational flow (its curl is always zero), but it is capable of describing a compressible flow.  We then apply the incompressible condition to solve the equations.  In approach 2, $\nabla \times \vec{\psi}$ is incpaable of describing a compressible flow (its divergence is always zero), but it is capable of describing a rotational flow.  We then apply the irrotational condition to solve the equations.


text:
I have looked on the Low altitude IFR chart for this information and can not find it listed
Inside the communications box after the name, what does (H), (H)(H), (L)(H), mean?

How do you know if a VOR is High, Low, or Terminal?

These are Standard Service Volumes, that denote how from how far away and how far up you can receive the navigational signal. (T) for terminal, (L) for low, and (H) for high.
There are also new service volumes, DL, DH, VL, and VH. You can find the exact definitions in the Chart Users' Guide, under "NAVAID STANDARD SERVICE VOLUME (SSV) CLASSIFICATIONS." As of October 5, 2023, it's on page 64.
For a combined navaid like a VORTAC, these service volumes don't need to be the same. The first listed is the VOR component and the second listed is the other component (e.g. DME for a VOR/DME).


text:
Medical requirements for becoming a Pilot
I’m currently a junior in high school and I would love to pursue being a comercial pilot as a career.
The thing is I have a medical condition called EoE. It’s basically when I eat something I’m allergic to, my esophagus swells up.
I’m currently treating it with dupixent injections. (1x a week)
Could having this medical condition stop me from getting my medical certificate?
Thank you,

@RTO With the relevant part quoted, that would make a good answer.

I have EoE that I am treating weekly with dupixent, and have a special issuance for a 3rd class medical. It's unlikely to be a problem as long as you have no side effects, but you can expect that the FAA will take some time to make a decision.
Monoclonal antibodies like dupixent require a special issuance with annual monitoring. Basically every year you will need to mail some clinical notes to the AMCD that says dupixent has still not made you blind or anything.

Thank you for your response.  How long did it take you to get the 3rd class medical including the special issuance? Also how much did the whole process cost you? Thank you,

@CoolPilot Just the cost of the third class physical. The FAA just wanted me to mail them some info from my doctors. I already had been the the doctor recently enough that they accepted letters from them without requiring another visit. It took several months for the FAA to make a decision, and my special issuance requires annual updates from my doctors.


text:
Type of airfoil used by Briggs, Hull, and Dryden
What airfoil do Briggs, Hull, and Dryden use in NACA Report No. 207: https://ntrs.nasa.gov/api/citations/19930091273/downloads/19930091273.pdf
They mention it a few times:
"The airfoils tested were members of a series of propeller sections of the form adopted by the engineering division of the Air Service as standard for propeller design. Six airfoils were used, the camber ratios, or ratios of maximum thickness to chord, being 0.10, 0.12, '0.14, 0.16, 0.18, and 0.20, respectively. The chord length was 3 inches and the span 17.2 inches so that the airfoils extended entirely across the jet, and there were no ends exposed to the stream as in ordinary wind-tunnel practice. The dimensions of the airfoils as measured by the gauge section of the Bureau of Standards are given in Table 1." They have a table of the dimensions of the airfoil as well.
I wasn't able to find the exact model of airfoils that they used. Here is the study: https://ntrs.nasa.gov/api/citations/19930091273/downloads/19930091273.pdf
Thanks in advance.

You can sort of go through airfoil design progress 1917-1947 by observing prop foil cross sections from hub to tip.  Even in the 1920s, aircraft engine power had progressed to the point of making prop tips go supersonic at full throttle.

Although there is no specific reference given to the propeller sections used in this report, by cross-referencing and checking several reports on propeller performance being completed at this period of time, one propeller appears in common.  The propeller section used in these reports was the RAF no. 6, modified.  The studies examining propellers using this section were all by Fred E. Weick, initially at the Bureau of Aeronatics, U.S. Navy, and later at the NACA Langley Memorial Aeronautical Laboratory.
The reports by Weick briefly examined were the following -

NACA Technical Note no. 212, Simplified Propeller Design for Low-Powered Airplanes, January 1925.

NACA Technical Note no 238, A Simplified Method for Determining the Strength of Propellers, January 1926. $^1$

NACA Technical Note no 244, Navy Propeller Section Characteristics as Used in Propeller Design, August 1926.

NACA Report no. 302, Full Scale Tests on a Thin Metal Propeller at Various Tip Speeds, January 1929.  $^2$

NACA Report no. 340, Full Scale Wind Tunnel Tests on Several Metal Propellers having Different Blade Forms, January 1931.  $^3$

$^1$  This report mentions the RAF 6 section, with tell-tale table of coordinates.
$^2$  This report specifically references NACA Report no. 207, the study by Briggs, Hull, and Dryden.
$^3$  This is a definitive study mentioning the RAF 6 section.
The study referenced in your question was evidently completed by the National Bureau of Standards.  A common practice in executing such studies was to use materials and methods already available that were used elsewhere.  In this particular case, the study may have been contracted to the National Bureau of Standards by the National Advisory Committee for Aeronautics.
And, as a further note regarding these types of propellers, making them was not easy.  The above referenced studies primarily concern tests of aluminum propellers.  Such propellers were made of a material called duralumin.  Propeller blades were forged first as a duralumin blank having the desired dimensions from which the propeller blade could be acquired.  This blank was then precision machined to the correct dimensions.  Because the cross-section of the propeller blade varied along the length of the blade, specific design of the cross section was not a primary consideration.  However, reproducibility was primary.  Consequently, a desired blade cross section would be of an airfoil that was easily manufactured, adequate for producing desired lift, and reproducible with precision.  The RAF 6 section apparently meets these needs, exactly.
This was an interesting question.  Thanks for asking.

The paper is almost one hundred years old (it's dated March 1925). According to Wikipedia the NACA airfoil system was introduced later: "By 1929, Langley had developed this system to the point where the numbering system was complemented by an airfoil cross-section, and the complete catalog of 78 airfoils appeared in the NACA's annual report for 1933."
Table I in the paper contains anyway the coordinates of the airfoils used in the test. Plus, the same airfoils had been used in the NACA "Report No 83" which also gives some information about the airfoils.
Hope this helps


text:
What determines the name of the runway if it is not active (for atc instructions)
I was not succesful with finding an answer to this anywhere on the web. Let’s assume we have multiple runways in the airport and ATC controller gives us instructions to cross certain runway that is not active at the moment.
The runway they mention is not active so its name does not have to be determined by wind direction (or as in use).
Let’s say we have two active runways 22L and 22R and ATC controller wants us to cross runway 31L/13R. Which of these names is used while in common only one name is used at a time?
I was listening liveatc.net from JFK when the ATC controller asked the pilot to cross 31R but the pilot readbacked with 13L. Obviously it´s the same physical runway but what is legit / correct / right to choose first?

It's pretty much dealer's choice (or controller's choice) how to refer to an inactive runway. They might choose to refer to the runway according to the most commonly used direction or calm-wind-use direction as that's what they normally say and it flows better. Or as @Michael Hall said they may change it up based on which landing threshold the pilot is nearest to.
In fact, at my airport I will "switch it up" for one of the runways (depending on where a landed aircraft is going it is common for me to have told one of them to "cross runway 36" and another "cross runway 18") but the other runway I will almost always refer to by its "normal" name of 27, because it is so rarely used as runway 9.
Of course if the runway is temporarily active then I will refer to it according to its active direction, so that everyone is on the same page.

I would think it would depend on the specific layout of runways at that airfield, where the wind is coming from in general, and which end of the runway you are closest to at the point of crossing.
Consider for example the airport below:  If RW 35 was in use and you were leaving from the Air Carrier Terminal at the NW corner you would be given instructions to taxi south on W, and would likely be cleared to cross RW 8, simply because that is the approach end numbers for that runway.
However, if you were coming from the hangars to the NE and were to cross the same 8/26 runway at midfield on taxiway F, I'm not sure what the controller would say with absolute certainty. However, there may be a technigue they teach at ATC school, and/or a local standard to ensure clarity.  In any case, the clearance should be unambiguous.  Read it back verbatim, and ask if you are uncertain.


text:
Why do airline pilots have shoulder harnesses?
On airliners, passenger seatbelts are simple lap belts. However, flight crews seem to have at least shoulder straps, and often five-point harnesses. Why do flight crew have more restraints?

Is your question about why do pilots have 5 point harnesses, or why don't passengers?

pilots need to be able to do work during turbulence.
passengers don't really like to be fully restrained.

I believe the primary reason for a shoulder harness is in case the pilot passes out and falls on the yoke

@Ben I don't believe that is true. Like a race/performance car driver, a pilot cannot perform his duties while displaced from his seat. Any reaction great enough to pull his feet off of the pedals, or hand from the control device (be it yoke, or fbw joystick) means the plane continues to fly out of control. Bad. Bad bad bad.

@CGCampbell Yes but a lap-style restraint can control that well enough. I'm talking specifically about the requirement for a shoulder harness.

@ben No, it cannot. Find your nearest car enthusiast and ask to take a ride. Get him to take you for a ride and go around a sweeping left turn. While in the turn, reach for the gear shift while your trunk/torso is pinned to the door. Now consider that a plane deals with three dimensions of turbulence often. Any time a pilot cannot safely reach whatever he is attempting to is a very bad thing.

@CGCampbell OK I take your point, I do see that an upper body harness provides an extra level of safety where it is needed most. Obviously one of those safety benefits does include the event of pilot incapacitation too! My line of thinking comes from an acquaintance who says he has a restriction on his medical that he must have a harness (although I thought harnesses were required anyway... maybe the light aircraft that he flies are different)

I'd really like to see some of these comments worked into the answer.

@Ben: No, a simple lap belt can't always control displacement from seat/controls.  I once hit unexpected serious turbulence in my Cherokee, which had only lap belts.  Wound up with the belt down around my thighs, and my head hit the roof hard enough to make me see stars.

@jamesqf - I heard of a king air pilot doing pipeline surveys who hit some CAT and hit his head, knocking him unconscious because his harness disconnected. (He apparently woke up with the plane crash-landed and had the engines running)

@Ben Shoulder belts do not need to be worn (and usually aren't) during flight. Taxi, takeoff and landing only. I did put mine on in turbulence also.

Even if the pilot could hold himself adequately in place and reach the controls do you want him fighting the turbulence or flying the plane?

They're required.
ICAO Annex 6, Part 1 states:

6.2.2 An aeroplane shall be equipped with:
  ...
  c) 3) a safety harness for each flight crew seat.  The safety harness shall incorporate a device which will automatically restrain the occupant's torso in the event of rapid deceleration

It also confirm's Ben's comment that they are also meant to restrain an incapacitated pilot:

Recommendation.– The safety harness for each pilot seat should incorporate a device to prevent a suddenly incapacitated pilot from interfering with the flight controls.

if there is something downvote-worthy in my answer, i'd appreciate your input.

While shoulder harnesses might prevent the body of a slumped-over pilot from touching the yoke, it doesn't do too much if their hand is on a sidestick and pushes it forward, does it?

I do not think the recommendation with respect to restraining an incapacitated pilot indicates that is the primary purpose. In fact, this is a recommendation, while restraint under deceleration is required. Both concerns look to me to be targeted to inertia-style belts, as ordinary shoulder belts meet these requirements automatically, without 'devices' - are inertia belts used, or even allowed? (BTW: I did not down-vote.)

@robokaren with sidesticks, input can be overridden (see http://aviation.stackexchange.com/q/3455/6919). much harder to override actual movement on the opposite yoke.

I recall early in my glider training when I was about 17 before Id gone solo.  My instructor used to keep his shoulder straps loose so I started doing the same.
On my 1st actual cable snap on the winch tow I whacked the stick forward and the glider went into -ve g.  It was an open-cockpit glider with no canopy and I was half hanging out and only just managed to reach and get fingertips of one hand onto the joystick to pull myself back into the plane.  I stabilised the glider, pulled my straps tight, but then found out my instructor behind me was not responding.  I landed safely and then found out he was unconscious.  He had bashed his head on the wing above him and passed out.
I always had my straps fastened properly after that incident.

Great anecdote, and I'm glad you both made it out alive! Unfortunately, not really an answer.

There have been some "unusual attitude" instances where the crew have recovered the aircraft but would have possibly been thrown from their seats or unable to reach the controls through excessive G if not fully restrained.

China Airlines 006 is actually a bad example for this, from the final report: "not all of those interviewed could recall the events of the upset, the descent, and the recovery. Most of those who could recall said that they felt an initial period of moderate negative G forces lasting several seconds followed immediately by a period of stronger positive G forces lasting several seconds. The positive G forces decreased momentarily and was followed by a period of even stronger positive G forces lasting several minutes."

In airliners, shoulder harnesses are not installed for passengers because:

it is not not mandated by law
it is too much for average passengers to deal with
it requires money to install
it is superfluous in a crash when compared to the same safety from a lap belt

Flight crew is required to use the shoulder harness (when installed) as it is the law (14 CFR 91.105):

(a) During takeoff and landing, and while en route, each required
  flight crewmember shall—

(1) Be at the crewmember station unless the absence is necessary to
    perform duties in connection with the operation of the aircraft or in
    connection with physiological needs; and
    (2) Keep the safety belt fastened while at the crewmember station.

(b) Each required flight crewmember of a U.S.-registered civil
  aircraft shall, during takeoff and landing, keep his or her shoulder
  harness fastened while at his or her assigned duty station. This
  paragraph does not apply if—

(1) The seat at the crewmember's station is not equipped with a shoulder harness; or
    (2) The crewmember would be unable to perform required duties with the shoulder harness fastened.

This only kind of deflects the question, though, doesn't it? The question just becomes "Why does the law mandate shoulder harnesses for pilots?" / "Why doesn't the law mandate shoulder harnesses for passengers?" which remains unanswered.

I'd like to see a reference for "harness...is superfluous in a crash when compared to the same safety from a lap belt", in cars a full harness is superior to a single shoulder belt,  and a single shoulder belt is superior to no shoulder belt (even with airbags). So I'd be really surprised if a shoulder harness provides no benefit in a plane crash. (though I can believe that since plane crashes are so rare, the expense and inconvenience is not worth it)

@Johnny: IIRC, a lap+shoulder belt is superior to a single shoulder belt, but a single shoulder belt is worse than no belt.

If the plane is in turbulence, and the passenger is jostled about, injured, or even knocked unconscious, well, sucks for him, but he will recover.
The more extreme the situation, the more important it is that the pilot be conscious, uninjured, and stable.

The main function of a shoulder harness or five-point restraint (as opposed to a simple lap belt) is to provide upper-body restraint and keep the occupant's head and upper torso from flailing about during rapid decelerations.
If an airliner's about to crash, the passengers can be told to assume a brace position, which likewise keeps the upper body from flailing about, making a simple lap belt sufficient for safely restraining the passenger.  The pilots, who have to (try to) fly the plane (or at least make sure it crashes as safely as possible), can't brace themselves, as this would prevent them from (e.g.) manipulating the flight controls; additionally, the area immediately in front of the pilots contains lots of potentially-injurious things, like the control yokes and various switches, buttons, dials, and levers, that the pilots' heads and upper torsos would smash into in the absence of upper-body restraint.
As a result of the more-injurious environment in front of the pilots and their inability to assume a brace position to protect their upper bodies in a crash, the pilots have restraint systems that provide upper-body restraint, such as shoulder harnesses or five-point restraints.

For passengers, while flying I feel fully restrained around the hips by a lap belt fully tightened- it would be more comfortable to have the belt load spread out. A full car seat belt compared to a lap belt allows you full motion.   Its more likely the cost and difficult of having more anchor points on all seats especially when they spread them across the plane- a place to join the belt to the seat at higher point. They might also have to design the seat to handle its load being more spread out. Let alone having to work out if having such "improved" belts actually helps survivability with the greater risk of being unable to unclip.

In car the shoulder strap is attached to the frame beside (or behind in the rear) the seat, so it can take very high loads. But in an airplane no similar attachment point is available in the passenger cabin (nor in a bus; some buses have belts for passengers, but they are also only lap belts).

@JanHudec Longer-distance buses in the UK are obliged to provide seatbelts by law. As I recall, the ones I take to the airport have full car-style seatbelts, not just lap belts.

@DavidRicherby: Hm, I've only ever seen the lap ones here.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Is turbulence behind an airliner less severe during cruise?
The turbulence and tip vortices behind an airliner are a familiar hazard for smaller aircraft when near an airport.  But a lightweight glider seems to have not suffered from this, when its tail was actually struck by an Airbus A 320 at altitude.  Page 8 of the accident report mentions only the choc brutal felt by the glider's occupants, and nothing about being blown around like a leaf.
At the time of collision, the airliner was descending through 8600 feet at a true airspeed of 280 knots, if my French can be trusted.  The glider was flying at 53 knots, almost perpendicular to the airliner's path.  So the glider would have encountered the airliner's wake for a good fraction of a second.  Its long wings would be quite sensitive to turbulence.
Compared to the Airbus's high thrust at takeoff, or slow speed and high drag at landing, would its turbulence be less hazardous here, at low thrust and high speed?
https://www.skybrary.aero/index.php/Wake_Vortex_Turbulence#Accidents_and_Incidents mentions some "en-route" turbulence, but only from one aircraft trailing another.  So maybe a cross-track incident is much milder.

I'm not sure an unprepared pilot can distinguish between severe wake turbulence and being struck by an actual airliner...

The forces transmitted by mechanical impact surely differ from those transmitted by turbulence to long skinny wings?

@CamilleGoudeseune - BTW, wake turbulence or wing tip vortices are the most severe when the aircraft is at its heaviest and slowest while in a “clean” configuration. This is usually at takeoff. High thrust really would not be a factor unless you are directly behind the engine. Thrust forces would dissipate in a lot shorter time and distance. Whereas wake turbulence can stick around for minutes. And, although the wing tip vortex will descend and spread outward away from the wingtip, it would be almost non-existent directly below the wing tip.

Interestingly enough, a Cessna 172 in a 30 degree bank can circle around onto its own flight path and hit its own wake turbulence in about a minute. It would not hit its own thrust forces.

I used to commute to work in my plane and one morning I was approaching my destination airport (uncontrolled FSS only, but with heavy freighters coming and going in the early hours) and was about 3 miles south of the approach that had been used by a DC-10 freighter that had landed several minutes before, planning to get on a downwind to land the opposite way. I'm pretty sure I passed through remnants of his wake several miles south of the approach path because in the dead smooth air there was a sudden jolt like driving over a curb, then smooth again. That's the only thing that makes sense.

@DeanF, makes sense.  Takeoff has more fuel weight and less flap deflection than landing.  @ John K, your "driving over the curb" sounds just like the incidents mentioned by Skybrary.

To answer the question directly: Yes, in cruise turbulence is less severe, but it is lowest when flying low and fast.
We can actually calculate the downward component $v_z$ of the wake (which is a good proxy for turbulence intensity) in cruise and during approach when we know how much lift is being created:
$$m\cdot g = \bar{m}\cdot v_z = \epsilon\cdot\frac{b^2}{4}\cdot\pi\cdot v_{\infty}\cdot\rho\cdot v_z$$
$$v_z = \frac{m\cdot g}{\epsilon\cdot\frac{b^2}{4}\cdot\pi\cdot v_{\infty}\cdot\rho}$$
The strength of the downwash is inversely proportional to the product of flight speed and density. This is not the dynamic pressure, which is proportional to flight speed squared times density, so at the same dynamic pressure the downwash gets stronger the higher you fly. This means a descending A320 at only 8000 ft should have a comparably weak wake.
Nomenclature:
$\kern4mm m\kern6mm$airplane mass
$\kern4mm g\kern7mm$gravitational acceleration
$\kern4mm \bar{m}\kern6mm$mass flow of air which is accelerated downwards
$\kern4mm v_{\infty}\kern4mm$flight speed
$\kern4mm b\kern7mm$wing span
$\kern4mm \rho\kern6mm$air density
$\kern4mm \epsilon\kern6mm$Oswald factor

Would I be correct (enough) in oversimplifying, that the amount of downwash, and tip vortex "strength" is always the same, but when going faster it just spreads out over larger volume of space, thus presenting itself to a bystander as lower in energy? (excluding flying close to ground, of course)

Would $v_z\cdot\rho$ be a better proxy?  Thinner air exerts less force.

@Jpe61 going faster means higher mass flow over (and under) the wing. Spreading the energy out is one thing, the other is that less energy is needed because the amount of wake deflection goes down with increasing speed. That is why induced drag is inversely proportional to the square of airspeed.

@CamilleGoudeseune: $v_z\cdot\rho$ is removing altitude dependency but adds one step to the explanation. So it is better indeed once the dependency has been explained.

The glider in question was "in touch" with the tip vortex of the A320 only with it's tail, and very briefly:

Picture from section 1.16.2 on page 24 of the
Accident investigation report.
The glider was flying at 53kts at almost a perpendicular angle to the path of the A320, and the tail of it struck the outer third of the leading edge of the Airbus. The G103 is about 8 meters long, a wing of an A320 is about 15 meters long, so the wings and most of the gliders fuselage were never superimposed by the wing of the A320:

Picture from section 2.4 on page 39 of the report.
As the glider was traveling across the path of the A320 at almost 27 meters per second, even its tail was out of the tip vortex in a blink of an eye, and in any case, only its tail ever was in the effect of the tip vortex. As the tip vortex forms at the tip of the wing, it's diameter is initially very small, growing larger as the distance to the plane grows.
Although the small tip vortex has high energy (high velocity), as the tail of the glider was very briefly traveling through the tip vortex, it was actually traveling in pretty much the same direction as the flow of the tip vortex was. So no great forces were exerted on the tail (except for the collision with the wing, of course...).
As such this incident can not be used as an example of the turbulence or tip vortex strength during cruise (or descent, to be precise), but the the said phenomena are less prominent at higher speed and lower angles of attack (normal cruise / cruise descent). They are at their strongest during slow flight when a high angle of attack is required to maintain a desired flight path (this would also apply for situations of high turn rate).

Thanks @FreeMan , sloppy writing from me...

Thanks, I'd overestimated how quickly turbulence would spread spanwise beyond the wings' span.  Wind tunnel photos of smoke trails, or CFD renderings, are rarely shown from above.

Btw, there are so many lucky ppl in that incident... a very close shave!

no worries, @Jpe61, that's what friends are for! ;)